1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more specifically, it relates to a method of fabricating a semiconductor device having an element isolation trench.
2. Description of the Prior Art
As the degree of integration of a semiconductor device is improved, a technique of finely working the semiconductor device becomes more and more important. An element isolation technique for isolating semiconductor elements forming the semiconductor device from each other is known as such a fine working technique. In relation to this element isolation technique, a method referred to as trench isolation is frequently employed as the degree of integration is improved.
In this trench isolation, an element isolation trench is formed on a semiconductor substrate and an insulating material or the like is embedded in the formed trench, thereby isolating element regions located on both sides of the trench from each other.
The aforementioned trench is formed by etching the semiconductor substrate. A corner portion is defined on an opening upper end of the trench formed by etching. An electric field or stress is concentrated at this corner portion, to disadvantageously deteriorate the characteristics of the semiconductor device. In general, therefore, a method of forming the trench and thereafter rounding the corner portion on the opening upper end of the formed trench by thermal oxidation thereby preventing the corner portion from concentration of an electric field or stress is proposed.
The aforementioned thermal oxidation for rounding the corner portion located on the opening upper end of the trench must be performed at a high temperature. However, when the temperature in an apparatus for thermal oxidation is previously increased to a level for performing thermal oxidation and the semiconductor substrate is thereafter inserted into this apparatus, for example, the semiconductor substrate may disadvantageously cause warpage or slip dislocation.
In order to suppress such warpage or slip dislocation of the semiconductor substrate, therefore, a method of inserting the semiconductor substrate into the apparatus for thermal oxidation, which is kept at a relatively low temperature, and thereafter increasing the temperature in the apparatus is proposed.
In this proposed method, however, the semiconductor substrate is disadvantageously oxidized before the temperature reaches a level sufficient for rounding the corner portion on the opening upper end of the semiconductor substrate. An oxide film formed under such a low temperature hardly flows in low-temperature oxidation due to a high coefficient of viscosity. When oxidation is performed under a low temperature, therefore, stress is readily applied to the semiconductor substrate due to volume expansion following oxidation. Particularly on the corner portion of the opening upper end, the semiconductor substrate is oxidized from two directions, i.e., upper and side directions, and hence stress applied to the opening upper end is larger than that applied to the remaining portions. The stress applied to the opening upper end is increased in proportion to the thickness of the oxide film formed at a low temperature. When the stress applied to the opening upper end is increased, the oxidation rate (oxidizing velocity) is so disadvantageously reduced that an overhanging point is formed on the corner portion of the opening upper end. In other words, the corner portion of the opening upper end is pointed in the form of an overhang when the thickness of the oxide film formed at a low temperature is in a large ratio to the thickness of the desired thermal oxide film. Even if oxidation is performed in a high-temperature region sufficient for relaxing stress by viscous flow, therefore, the quantity of oxidation is relatively reduced and hence the corner portion of the opening upper end cannot be sufficiently rounded.
An object of the present invention is to provide a method of fabricating a semiconductor device capable of sufficiently rounding an opening upper end of an element isolation trench.
Another object of the present invention is to perform high-temperature oxidation while relaxing stress applied to a semiconductor substrate in the aforementioned method of fabricating a semiconductor device.
Still another object of the present invention is to suppress thermal nitriding that may be caused when the temperature of a semiconductor substrate is increased to a prescribed level or etching with an extremely small amount of oxygen in the aforementioned method of fabricating a semiconductor device.
A further object of the present invention is to suppress formation of a thermal oxide film when the temperature is reduced after formation of an oxide film in the aforementioned method of fabricating a semiconductor device.
A method of fabricating a semiconductor device according to an aspect of the present invention comprises steps of forming an element isolation trench on a semiconductor substrate, performing thermal oxidation on at least an opening upper end of the element isolation trench while increasing the atmosphere temperature of the semiconductor substrate beyond a prescribed temperature thereby forming a first oxide film and suppressing formation of the first oxide film on the opening upper end before the atmosphere temperature is increased beyond the prescribed temperature.
In the method of fabricating a semiconductor device according to this aspect, formation of a first oxide film having a large thickness in a low-temperature region can be prevented by suppressing formation of the first oxide film on the opening upper end before the atmosphere temperature is increased beyond the prescribed temperature as described above, whereby stress applied to the opening upper end can be relaxed. Thus, thermal oxidation in a high-temperature region can be performed while relaxing stress applied to the opening upper end, whereby reduction of the oxidizing velocity can be prevented. Consequently, the opening upper end of the element isolation trench can be sufficiently rounded.
In the method of fabricating a semiconductor device according to the aforementioned aspect, the step of suppressing formation of the first oxide film preferably includes a step of increasing the temperature of the semiconductor substrate to the prescribed temperature in a non-oxidizing atmosphere thereby suppressing formation of the first oxide film. Thus, the first oxide film can be readily inhibited from formation under a low temperature below the prescribed temperature.
In this case, the method of fabricating a semiconductor device preferably further comprises a step of forming a second oxide film having a smaller thickness than the first oxide film in advance of the step of suppressing formation of the first oxide film. Thus, it is possible to suppress thermal nitriding that may be caused when the temperature of the semiconductor substrate is increased to the prescribed temperature in the non-oxidizing atmosphere or etching with an extremely small amount of oxygen. When a nitrogen atmosphere is employed as the non-oxidizing atmosphere, the surface of the substrate is disadvantageously roughened due to nitriding (thermal nitriding). When the non-oxidizing atmosphere contains an extremely small amount of oxygen, the surface of the substrate is not oxidized but etched with the extremely small amount of oxygen. Thus, the surface of the substrate is roughened. According to the present invention, the aforementioned thermal nitriding or etching can be suppressed by previously forming the second oxide film having a small thickness on the surface of the substrate. The aforementioned etching with an extremely small amount of oxygen is disclosed in xe2x80x9cUltraclean, integrated processing of thermal oxide structurexe2x80x9d, Appl. Phys. Lett. 57, No. 12, Sep. 17, 1990, pp. 1254-1256 etc.
In this case, the step of forming the second oxide film preferably includes a step of forming the second oxide film in an atmosphere prepared by adding a small amount of oxygen to non-oxidizing gas consisting of at least either rare gas or nitrogen gas. Thus, the second oxide film having a small thickness can be readily formed. In this case, the concentration of the oxygen is preferably at least about 0.2% and not more than about 25%. Further, the second oxide film preferably has a thickness of at least about 2.0 nm and not more than about 6.0 nm.
In the method of fabricating a semiconductor device according to the aforementioned aspect, the aforementioned prescribed temperature is preferably a temperature reducing the coefficient of viscosity of the first oxide film formed by the thermal oxidation below a prescribed value. Thus, the formed first oxide film readily causes viscous flow, whereby the semiconductor substrate can be effectively prevented from application of stress in high-temperature oxidation. In this case, the prescribed temperature is preferably at least about 950xc2x0 C. and not more than about 1150xc2x0 C.
In the method of fabricating a semiconductor device according to the aforementioned aspect, the step of forming the first oxide film preferably includes a step of performing thermal oxidation in an oxygen atmosphere prepared by diluting oxygen with non-oxidizing gas consisting of at least either rare gas or nitrogen gas while increasing the temperature of the semiconductor substrate beyond a prescribed temperature thereby forming the first oxide film. Thus, the amount of formation of the first oxide film can be more correctly controlled. In this case, the concentration of the oxygen is preferably at least about 2.0% and not more than about 35%. Further, the first oxide film preferably has a thickness of at least about 10 nm and not more than about 50 nm.
In the method of fabricating a semiconductor device according to the aforementioned aspect, the step of forming the first oxide film preferably includes a step of performing thermal oxidation in an oxygen atmosphere containing halogen-based gas while increasing the temperature of the semiconductor substrate beyond a prescribed temperature thereby forming the first oxide film. When thermal oxidation is performed in the oxygen atmosphere containing halogen-based gas, the diffusion coefficient of an oxidant is so increased that rounding/oxidation of the opening upper end can be facilitated. Further, the temperature for properly rounding/oxidizing the opening upper end can be reduced due to such facilitation of oxidation.
In this case, the halogen-based gas may include Cl-based halogen gas, and the concentration of the Cl-based halogen gas with respect to an oxidant may be at least about 1.0% and not more than about 10%. Alternatively, the halogen-based gas may include F-based halogen gas, and the concentration of the F-based halogen gas with respect to an oxidant may be at least about 0.0005% and not more than about 0.1%.
The method of fabricating a semiconductor device according to the aforementioned aspect preferably further comprises a step of reducing the temperature in a non-oxidizing atmosphere after forming the first oxide film. Thus, formation of a thermal oxide film can be suppressed during temperature reduction. In this case, the step of reducing the temperature preferably includes a step of reducing the temperature to about 650xc2x0 C. to about 800xc2x0 C. in a non-oxidizing atmosphere.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.